20101010

Astronomy quiz question: sidereal motion (2)

An observer in San Luis Obispo, CA notices that the constellation Sagittarius is just above the southwest horizon at 9:00 PM on October 5. (Ignore the fact that your starwheel does not include daylight saving time.) Exactly 24 hours later, this constellation will be __________ the southwest horizon.(A) slightly lower above. (B) at exactly the same position above. (C) slightly higher above. (D) (None of the above choices, as it would not be visible anywhere in the sky.)

Correct answer: (A)

Using a starwheel (planisphere), the position of Sagittarius will shift slightly the next day, as seen by aligning 9:00 PM with October 5, and then aligning 9:00 PM with October 6, resulting in this constellation being slightly lower above the southwest horizon at the same time the next evening. This is caused by Earth's revolution around the sun while simultaneously rotating about its own axis. Response (B) would be true if Earth did not revolve around the sun, and only rotated about its own axis.